1. Field of the Invention
The present invention relates in general to a fluid-filled cylindrical elastic mount which provides a desired vibration damping or isolating effect based on flow of a fluid contained therein, and more particularly to such a cylindrical elastic mount that exhibits different vibration damping or isolating characteristics depending upon a frequency range of input vibrations.
2. Discussion of the Related Art
A cylindrical elastic mount is known as a vibration damper interposed between two members of a vibration system, to flexibly connecting the two members or flexibly mounting one of the two members on the other member. The cylindrical elastic mount includes an inner sleeve, an outer sleeve disposed radially outwardly of the inner sleeve with a suitable spacing therebetween, and an elastic body for elastically connecting these sleeves. In recent years, there is proposed a so-called fluid-filled cylindrical elastic mount having a pair of fluid chambers formed between the inner and outer sleeves, and an orifice passage through which the fluid chambers are held in fluid communication with each other. This elastic mount is adapted to damp vibrations applied thereto, based on the resonance of the fluid flowing through the orifice passage.
However, the known fluid-filled cylindrical elastic mount of the above type provides a sufficiently high damping effect with respect to a certain range of frequency of the input vibrations. That is, the damping of the input vibrations based on the resonance of the fluid flowing through the orifice passage is effective only to a predetermined frequency band to which the orifice passage is tuned. If the orifice passage is tuned so that the elastic mount provides an excellent damping effect with respect to vibrations in a relatively low frequency range, the amount of the fluid flowing through the orifice passage is almost negligible as if the orifice passage were closed, when the elastic mount receives vibrations in a relatively high frequency range. Consequently, the dynamic spring constant of the elastic mount is undesirably increased, and the vibration isolating characteristics of the mount are considerably deteriorated.
In view of the above situation, there is proposed in JP-A-2-245538 (published in 1990) a cylindrical elastic mount having a first and a second orifice passage which are formed independently of each other by closing respective grooves formed in an outer circumferential surface of the elastic body. The second orifice passage has a higher ratio of its cross sectional area to its length than that of the first orifice passage. This elastic mount further includes a gate valve mounted on the outer sleeve, which valve gets into or gets out of the second orifice passage so as to inhibit or permit flow of the fluid through the second orifice passage, respectively.
When the gate valve is operated to its closing position for inhibiting the fluid flow therethrough, the cylindrical elastic mount described above provides a sufficiently high damping effect for the low-frequency vibrations, based on the fluid flow through the first orifice passage. When the gate valve is operated to its open position for permitting the fluid flow therethrough, the elastic mount provides a sufficiently high isolating effect for the high-frequency vibrations, based on the fluid flow through the second orifice passage. Thus, the vibration damping or isolating characteristics of the elastic mount can be suitably controlled by operating the gate valve.
In the thus constructed fluid-filled cylindrical elastic mount, however, the gate valve is repeatedly brought into and out of the fluid in the second orifice passage, through the outer sleeve, making it difficult to ensure sufficient fluid-tightness at a portion of the mount where the gate valve is in sliding contact with the outer sleeve. In particular, it is extremely difficult to assure sufficiently high durability of the mount if the gate valve is required to frequently get into and out of the second orifice passage, through the outer sleeve.
Further, the outer sleeve of the fluid-filled cylindrical elastic mount of the above type needs to be formed with a mounting seat for supporting the gate valve. Therefore, the outer sleeve must be formed by die casting or other molding technique, resulting in reduced efficiency and increased cost in fabricating the elastic mount.